1. Field of the Invention
This invention relates generally to impregnation sealant compositions, comprising a curable monomer adapted for curing by heat and/or substantial absence of oxygen. More specifically, the invention relates to an impregnation sealant composition of such type, having superior high temperature resistance properties, and to a method of making same.
2. Description of the Related Art
Impregnation sealing of porosity in porous parts frequently is carried out by introducing sealant compositions into porosity under a pressure differential, by well known techniques, or by wicking methods in which the impregnation sealant is flowed across the surface of a porous part and allowed to enter into the voids thereof by capillary action during a selected period of time.
Such impregnation sealing of porosity is used in the manufacture of porous metal parts and other porous materials and structures, to avoid problems incident to microporosity. Such problems include air, gas and fluid leakage susceptibility, which may create processing or finishing problems, as well as difficulties in the end use of the porous member. Sealing of porosity thus is employed to render parts leak-proof in character and to prevent or minimize the incidence of internal corrosion, when metal parts are involved. The development of impregnation sealing compositions used in the production of castings, dicastings, electronic components, powder metal parts, sintered components, fiberglass composites, and other materials and structural elements continues to be a significant field of development.
The sealant compositions typically employed in the aforementioned impregnation applications include a wide variety of self-curing anaerobic sealants, which are curable via free radical polymerization in the presence of suitable free-radical initiators, e.g., peroxy-type initiators, as well as thermal (heat)-curing sealants, and sealants which cure by both anaerobic and heat cure mechanisms.
As used herein, the term "anaerobic" refers to substantial absence of oxygen, and sometimes hereinafter is referred to as an anoxic condition, in reference to the cure environment of a specific curable composition.
Illustrative of (meth)acrylic monomer-based anaerobic impregnant compositions known in the art are U.S. Pat. Nos. 3.672,942; 3,969,552; Reissue U.S. Pat. No. 32,240; and U.S. Pat. No. 4,632,945. Thermal-curing sealant compositions include the compositions described in U.S. Pat. No. 4,416,921 and U. K. Patent Specifications 1,308,947 and 1,547,801.
Anaerobic cure impregnation sealants have significant advantages including rapid cure time and good cured sealant properties, but typically are provided as two-part compositions, one including the monomer to be polymerized, and the other part containing a catalytic accelerator of polymerization, so that upon mixing of such parts, the composition is continuously aerated or otherwise maintained in an oxic (oxygen exposure) condition until polymerization is desired.
Heat-curing (meth)acrylic monomer-containing impregnant compositions, by contrast, may be effectively used with a minimum of monitoring and maintenance, with little or no aeration or oxic conditions being required. Such impregnant compositions may cure at temperatures ranging from 40.degree. C. to 150.degree. C., depending on the specific formulation employed.
Once the heat-curable impregnant composition is introduced into the porosity of the parts to be sealed, the parts typically are transferred to an agitated water rinse zone for removal of any remaining surface accumulations of sealant or extraneous sealant which is trapped in blind holes of the impregnated parts. After removal of the excess sealant, the impregnated parts are passed to a tank containing hot water, e.g., at a temperature of 90.degree. C.-200.degree. C., or other medium at elevated temperature which serves to cure the sealant composition in the porosity.
in the case of anaerobic cure impregnant sealant compositions, the impregnant composition is likewise introduced into the porosity of the parts to be sealed, and the parts are transferred to an agitated water rinse zone for removal of any excess surface accumulations of sealant or extraneous sealant which is trapped in blind holes of the impregnated parts. After such removal of excess sealant, the impregnated parts typically are passed to a tank containing a catalyst activator solution which serves to cure the sealant composition at the entrance to the porosity. This creates a hardened plug or cap in the outer portion of the pores, trapping the remaining resin for anaerobic self-cure.
A problem with curing of impregnation sealants, associated with heating of the impregnant sealing composition (for curing in the case of thermal cure compositions, and for accelerating cure in the case of anaerobic sealant compositions), is termed "bleed out," in which loss of resin from pores occurs due to a combination of opening of the pores with increasing temperature, thermal expansion of the sealant resin, and reduced viscosity of the resin at elevated temperatures. Such bleed out phenomenon causes a loss of sealing effectiveness, and also can account for sticky residues that sometimes are left on parts after curing of sealant.
In addition to the foregoing, both heat-cure and anaerobic sealants may be prone to thermal degradation during exposure to high temperatures, even though kinetically such elevated temperature may enhance the kinetic rate of polymerization of the monomer in the impregnant composition.
In general, there is a signifcant need in the art for porosity impregnation sealants which can function without adverse affect at high temperatures.
Although a number of high temperature resin systems have evolved in various application fields other than porosity impregnation, such as epoxies, phenolics, polyimides, polysulfones and the like, such resins have not been contemplated for porosity impregnation applications, for various reason, including viscosity characteristics, the extremely high temperatures which are required in some instances to effect curing thereof, and the comparatively long curing times required to reach substantially full cure properties. By contrast, (meth)acrylic resins have been almost exclusively used in porosity impregnation applications, due to their highly advantageous viscosity characteristics, and rapid curability in anaerobic cure and/or heat-cure formulations. In addition, the cure conditions required for heat-curing (meth)acrylic impregnation sealant compositions are relatively mild in contrast to polyimides, phenolics, polysulfones, and heat-curing epoxies.
Accordingly, it would be a significant advance in the art to provide a curable impregnation sealant composition having superior high temperature resistance when cured, as well as the good viscosity and rapid cure characteristics of (meth)acrylic resin-based systems currently in use.
Accordingly, it is an object of the present invention to provide such impregnation sealant compositions which utilize currently known (meth)acrylic resins in their formulations, and thus retain the rapid cure character of such resins, and which moreover have good viscosity characteristics which remain stable even under elevated temperature conditions in their curing, and which have superior high temperature resistance properties in the cured state.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.